The present invention relates to a device for occluding a defect through a spinal disc anulus. More particularly, it relates to an occlusion device adapted to provide a strong mechanical closure or barrier for a spinal disc anulus defect.
The vertebral spine is the axis of the skeleton upon which all of the body parts “hang”. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar segments sit upon a sacrum, which then attaches to a pelvis, in turn supported by hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints, but allow known degrees of flexion, extension, lateral bending and axial rotation.
The typical vertebra has a thick interior bone mass called the vertebral body, and a neural (vertebral) arch that arises from a posterior surface of the vertebral body. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch that extends posteriorly and acts to protect a posterior side of the spinal cord is known as the lamina. Projecting from the posterior region of the neural arch is a spinous process. The central portions of adjacent vertebrae are separated and supported by an intervertebral disc.
The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motions within vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: The nucleus pulposus (“nucleus”), the anulus fibrosus (“anulus”), and two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus serve to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae.
The anulus of the disc is a tough, outer fibrous ring that binds together adjacent vertebrae. This fibrous portion, which is much like a laminated automobile tire, is generally about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the anulus consist of 15 to 20 overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 30 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction, relative to each other. The laminated plies are less firmly attached to each other.
Immersed within the anulus, positioned much like the liquid core of a golf ball, is the nucleus. The anulus and opposing end plates maintain a relative position of the nucleus in what can be defined as a nucleus cavity. The healthy nucleus is largely a gel-like substance having high water content, and similar to air in a tire, serves to keep the anulus tight yet flexible. The nucleus-gel moves slightly within the anulus when force is exerted on the adjacent vertebrae with bending, lifting, etc.
Under certain circumstances, an anulus defect (or anulotomy) can arise that requires surgical attention. These anulus defects can be naturally occurring, surgically created, or both. A naturally occurring anulus defect is typically the result of trauma or a disease process, and may lead to a disc herniation. A disc herniation occurs when the anulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal anular confines. The mass of a herniated or “slipped” nucleus can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis.
Where the naturally occurring anulus defect is relatively minor and/or little or no nucleus tissue has escaped from the nucleus cavity, satisfactory healing of the anulus may be achieved by immobilizing the patient for an extended period of time. A more practical solution would be artificially obstructing or occluding the defect with an auxiliary device. Unfortunately, an effective anulus defect occluder able to maintain its position relative to an even minor anulus defect has not heretofore been developed.
A more problematic anulus defect concern arises in the realm of anulotomies encountered as part of a surgical procedure performed on the disc space. As a starting point, bed rest alone cannot adequately heal many disc herniations, such that a more traumatic surgical intervention is required. Alternatively, with discal degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases, causing the anulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur, which may contribute to persistent and disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain.
In many cases, to alleviate pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae surgically fused together. While this treatment alleviates the pain, all discal motion is lost in the fused segment. Ultimately, this procedure places greater stress on the discs adjacent the fused segment as they compensate for the lack of motion, perhaps leading to premature degeneration of those adjacent discs. A more desirable solution entails replacing, in part or as a whole, the damaged nucleus with a suitable prosthesis having the ability to complement the normal height and motion of the disc while stimulating the natural disc physiology.
The first prostheses embodied a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetic discs were designed to replace the entire intervertebral disc space, and were large and rigid. Beyond the questionable efficacy of these devices is the inherent difficulties encountered during implantation. Due to their size and inflexibility, these first generation devices require an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve rootlets during posterior implantation, could not be avoided. Recently, smaller and more flexible prosthetic nucleus devices have been developed. With the reduction in prosthesis size, the ability to work around the spinal cord and nerve rootlets during posterior implantation has become possible.
Generally speaking, these reduced size prostheses are intended to serve as a replacement for the natural nucleus. In other words, the anulus and end plates remain intact, and the prosthesis is implanted within the nucleus cavity. In order to implant a prosthesis within the nucleus cavity, an appropriately sized passageway through the anulus (i.e., anulotomy) must exist. The requisite anulus defect can be surgically imparted as part of the surgical implantation procedure, or the naturally occurring anulus defect that caused or resulted from the discal failure may be large enough for passage of the prosthesis. One pre-implant anulotomy technique entails complete removal of a plug of tissue from the anulus via an incision created by a scalpel, punch or similar tool. Entire removal of an anulus segment is highly traumatic, and limits the ability of the anulus to properly heal. Attempts to reattach the anulus plug have been unavailing in that properly orienting and securing of the anulus plug with a suture has proven difficult at best. Alternatively, a flap can be imparted into the anulus tissue. This technique overcomes the reattachment problems associated with the anulus plug approach. Unfortunately, however, the thickness of the anulus requires formation of a relatively large flap, therefore increasing anulus trauma. Further, it may be difficult to retain the flap in a retracted position throughout the implantation procedure. A third, more viable procedure is to dilate a small opening or incision in the anulus to a size sufficient for prosthesis implantation. The overlapping, plied nature of the anulus tissue renders the anulus highly amenable to incision dilation.
Regardless of the anulotomy technique, the resulting anulus defect may lead to post-implant complications. The anulus tissue will, in theory, regenerate or naturally repair the defect over time. However, substantial scar tissue formation will not occur for a significant period of time, and requires that forces on the spinal tract be minimized (i.e., that the patient be immobilized). For virtually all patients, this is impossible to achieve. Instead, within several days of the implantation procedure, the patient must move about, thereby placing forces on the disc space. Because the anulus defect has not healed, it cannot readily prevent the prosthetic nucleus, or portions thereof (depending upon the particular prosthesis construction), from migrating back through the anulus defect. Even if this opening is closed via sutures following implant, various forces acting upon the disc space have the potential to overcome the resistance provided by the sutures and “push” the prosthesis back through the anulus opening. A more preferable solution would be the provision of an auxiliary device that serves to not only occlude the surgically-created anulus defect, but also rigidly resists explant of the prosthesis, or portions thereof, back through the opening. Unfortunately, and as previously described, such a device has not heretofore been developed.
Spinal disc anulus defects occur both naturally and as part of a surgical procedure. Currently accepted techniques of suturing the defect closed are of minimal value in light of the forces normally encountered by the disc space. Even more problematic is the inability to protect against explant of a prosthetic spinal disc nucleus otherwise implanted through a surgical-imparted anulus defect. Therefore, a need exists for a spinal anulus defect occlusion device capable of effectuating anulus repair and providing a strong mechanical closure/barrier required for successful prosthetic disc nucleus implantation.